Project Summary Acute kidney injury (AKI) is associated with a high mortality in native kidneys, decreased allograft survival in transplants, and very high health care costs. Among the most common etiologies of AKI in both native and transplanted kidneys is ischemia-reperfusion injury (IRI). Despite advances in medical care, the mortality and morbidity associated with AKI after IRI remain high, with no specific therapy. A major unknown is the role of different T cell types in either causing/preventing tissue damage or improving/worsening repair. Detailed understanding of the roles of different T cell types holds great promise in both elucidating mechanisms underlying IRI and also in devising new strategies to ameliorate AKI. One such strategy is to shift the T cell balance in favor of protective T cell types and away from pathogenic ones. Therefore, it is important to study different T cell types residing and trafficking to the kidney and investigate their roles in the steady state and IR conditions. Our team has had a long-standing interest in studying immune cells in AKI using experimental animal models with translational potential. We recently discovered a unique population of CD4-CD8- double negative (DN) CD3+TCRab+ T cells that occupies a large niche of the ab T cell compartment in the normal mouse kidney and undergoes marked changes during AKI. However, their functions in the normal kidney and AKI pathogenesis remain relatively unknown. Based on strong preliminary data on DN cells, our overarching hypothesis is that kidney DN T cells play a major and unique role in regulating renal immune cell homeostasis in the steady state and after AKI. Our specific Aims are: 1) Investigate mechanisms and significance of steady state proliferation of kidney DN T cells. 2) Investigate immunoreactivity of DN T cells to IRI induced AKI. 3) Test the ability of DN T cells to prevent AKI and/or accelerate repair using gain and loss-of-function strategies. Establishing a functional role for this previously unappreciated cell type in the kidney will substantially advance our understanding of the kidney immunology and could lead to paradigm shifting scientific concepts.